standing tree height?

An older gent was telling me this morning that there was some type "rule of thumb" that could be used to determine the height of a standing tree. This method did not require tools. Any ideas out there as to the method?

walk away from the tree just to the point where you can see the top of the treelooking between your legs upside down when you can see it how far you are away from the tree is about the height of the tree

stand up agin tree.stick knife just above head . move back till fist and up raised thumb equal ground too knife .use second hand and repaet counting number of hands high multiply by your height . practice away from power line and remove knife before cutting!!

...................a squared = shadow cast by the heighth of tree
...................b squared = actual heighth of tree
...................c squared = the Hypotenuse of the right triangle .
.............<<or>>
..................Hire skinny kid to climb tree with 100 foot tape measure with lead weight tied to the end . this would be b squared . fordy..